The present invention relates in general to insulated hot fluid injection tubing and more particularly to a new and useful arrangement for maintaining a vacuum in an annular space between inner and outer tubulars forming an insulated tubing.
Heavy oil and tar sands represent huge uptapped resources of liquid hydrocarbons which will be produced in increasing quantities to help supplement declining production of conventional crude oil. The deposits must, however, be heated to reduce the oil viscosity before the oil will flow to the producing wells in economical quantities. A dominant method of heating is by injection of surface generated steam in either a continuous (steam flood) or intermittent (steam stimulation or "huff and puff") mode.
When steam is injected down long injection pipes or "strings", a significant amount of thermal energy is lost to the rock overburden (500 to 7,000 feet) which covers the oil deposits if the strings are not properly insulated. In initial steam injection projects, the price of oil did not justify the prevention of this heat loss, but with the price of oil at $30 or more a barrel, insulation systems for the well injection pipe become economically justified.
It is known to use insulated steam injection tubing for the injection of steam into oil wells and the prevention of excessive heat loss.
Tubing of the insulated steam injection type is formed of coaxial inner and outer tubulars that are connected together whereby an annular space is formed there-between. The annular spaces are typically insulated by products such as fiber and layered insulation with air or inert gas typically in the annular spaces.
The provision of a vacuum within an annular space between inner and outer tubulars is disclosed in U.S. Pat. No. 3,680,631 to Allen et al. and U.S. Pat. No. 3,763,935 to Perkins. Both of these references deal with the conveyance of warm fluid, such as oil over cool environments such as permafrost zones wherein the fluid, specifically liquid petroleum, is to be conveyed typically at a temperature of 160 degrees F.
Both of these patents suggest the use of special coatings, such as nickel or chromium alloy coatings, on the tubular surfaces to reduce gas diffusion into the space so that the vacuum which is originally established in the annular space can be maintained. Both patents generally suggest the use of a getter material for absorbing gases which may invade the annular space.
While the problem of gases diffusing or leaking into an evacuated annular space of a double walled tube is treated generally in the Allen et al and Perkins patents, neither of these patents address additional problems which are faced in the rugged environment of an oil well undergoing steam injection. The outer surfaces of the outer tubular in such an environment are exposed to corrosive water under pressure, which pressure increases with well depth. The tubing is generally made of carbon steel for economic reasons, and the corrosive environment drastically increases the generation of nascent hydrogen that permeates the outer tubular wall in particular at the greatly increased pressures encountered in typical water depths of 4000 to 6000 feet or more.
In addition, under the high temperature conditions of the inner tubular, the outgassing of objectionable gases such as oxygen, carbon monoxide, hydrogen, and nitrogen into the annular space increases in the order of an estimated ten times or more over the outgassing rate when the fluid in the inner tubular is at a temperature of merely 160 degrees F. Again, for economic reasons, the inner tubular should normally be made of relatively inexpensive metals such as carbon steel. While baking is known for the purposes of outgassing the surface of such steel, it is estimated that sufficient degassing of the inner tubular would require a temperature of 1,800 degrees F. for a period of about a day. Such processing is generally impractical, however.
During the life of an insulated steam injection tubing, which is estimated to be at least five years, an increase in hydrogen partial pressure within the annular space of up about four torr can be expected due to hydrogen diffusion. An increase in partial pressure from other active gases of 100 torr can be expected from outgassing of the inner tubular. Such increases in pressure defeat the insulating function of the annular space. Although partial pressures of other gases of up to 0.1 torr can normally be tolerated, a partial pressure above 0.01 torr cannot normally be tolerated for hydrogen due to its greater mobility.
In an article entitled New Double-Walled Tubulars Can Aid Thermal-Recovery Operations, B. V. Traynor, Oil and Gas Journal, Feb. 18, 1980, the problems of insulating the annular spaces of double-walled tubing in a rugged oil well environment are discussed on p. 106. It is noted that the use of a vacuum in the annular space for insulation purposes was found to be economically impractical so that the vacuum approach has been abandoned.